This invention relates to the production of low dielectric loss ceramic materials, particularly ceramic materials useful in dielectric resonators. In current microwave communication, technology dielectric resonators (DRs) are key elements for filters, low phase noise oscillators and frequency standards. DRs possess resonator quality factors (Q) comparable to cavity resonators, strong linearity at high power levels, weak temperature coefficients, high mechanical stability and small size.
Ceramic dielectric materials are used to form thermally stable DRs as key components in a number of microwave subsystems which are used in a range of consumer and commercial market products. These products range from Satellite TV receiver modules (frequency converter for Low Noise Broadcast (LNB)); Cellular Telephones, PCN""s. (Personal Communication Networks Systems) and VSAT (Very Small Aperture Satellite) systems for commercial application to emerging uses in transportation and automobile products, such as sensors in traffic management schemes and vehicle anti-collision devices. Dielectric Resonators may be used to determine and stabilise the frequency of a microwave oscillator or as a resonant element in a microwave filter. New systems of satellite TV transmission, based on digital encoding and compression of the video signals, determine the need for improved DR components. The availability of advanced materials will also enable necessary advances in the performance of DRs used for other purposes as referred to above.
Low dielectric loss materials are highly desirable in the area of communications over a wide frequency range. As an example, resonators using dielectric sintered ceramics may be used in base stations required for mobile communications. The materials used are often complex mixtures of elements.
One of the earliest resonator materials was Barium Titanate (BaTiO3 or BaTi4O9 see, for example, T Negas et al American Ceramic Society Bulletin, vol. 72, pp 88-89 1993). The dielectric loss of a material is referred to as the tan delta and the inverse of this quantity is called the Q (Quality Factor). The Q factor of a resonator is determined by choosing a resonance and then dividing the resonant frequency by the band width 3 dB below the peak.
The losses in ceramic materials may be associated with molecules or defects which can be spatially oriented (Debye loss), due to the inertia of free charges, e.g. electrons in a metal or resonant absorption at certain frequencies. It is considered that extrinsic factors such as impurities and e.g. oxygen vacancy concentration as well as microstructure are of overriding importance. Single crystals or xe2x80x98perfectxe2x80x99 crystals have a massively lower loss then corresponding poly-crystalline materials. The difference between a xe2x80x98perfectxe2x80x99 single crystal and a polycrystalline ceramic are thought to be due to the huge differences in microstructure and perfection between the two and are clear indicators why it is considered impossible to achieve a dielectric loss approaching that of single crystal counterparts in sintered materials.
The particulate ceramic material can be shaped in a variety of ways, for example, by uniaxial powder pressing, by isostatic pressing, by slip-casting or by polymer processing and extrusion. The resultant shape is then sintered at high temperature and this is associated with a shrinkage and a decrease in the volume of the body. In the prior art, the sintering step can take place in air or in special atmospheres be they oxidising, reducing or inert.
Sintering a ceramic involves taking a fine powder of the material, pressing it into the desired shape and then heating it to temperatures less than their melting point (usually about 75% of the melting point). The powders sinter together in an effort to reduce surface energy and this is accomplished by the reduction in surface area until the porosity is reduced substantially or entirely. The sintering process involves less expensive capital equipment and is less energy intensive than for forming single crystals.
A single crystal is made from a melt and the melting temperature of alumina is 2072xc2x0 C.
The major problem with dielectric ceramics is that their dielectric loss is much higher than single crystals. Single crystal materials can exhibit very low loss and this is usually attributed to the absence of grain boundaries and the greater perfection in their structure.
The problem with single crystals is that they are time consuming to manufacture and they are extremely expensive. For example, a single crystal of alumina in cylindrical form is around 10,000 times more expensive than an identically shaped sintered alumina.
A material which has been used for DRs is sintered alumina and this has been found to have a Q factor very much less than a single crystal.
In order to obtain dense alumina ceramics it can be necessary to add binders such as polyvinyl alcohol or microcrytalline wax to the powders or to treat the alumina powder before sintering, however the addition of the binder or the pre-treatments used have reduced the Q value to too low a value.
U.S. Pat. No. 3,637,406 discloses alumina of fine grained structure, however the alumina structure disclosed in this patent incorporate organic binders such as polyvinyl alcohol which adversely affect the Q value so that the Q value is reduced.
EP 0678489 discloses a high hardness alumina product, the alumina structures disclosed in this patent are obtained by sintering alumina powders, but the ceramics disclosed are formed by the addition of a binder or pre-treating the alumina powder with nitric acid.
A primary object of the invention is to provide an alumina with useful for making DRs with a lower dielectric loss.
This object can be obtained by means of a sintered alumina having a Q value greater than 30,000 at 9-10 GHz and at 25xc2x0 C.
Preferably the alumina has a porosity less than 2% (density greater than 98% of the theoretical value), and more preferably less than 1% (density greater than 99% of the theoretical value).
Preferably the temperature of sintering for powders such us alumina is less than 1600xc2x0 C. And more preferably between 1500xc2x0 C. and 1600xc2x0 C.
The sintered alumina of the present invention can be made in any conventional way, e.g. by pressing the powdered alumina into a shape and then heating to a temperature below its melting point. The powders sinter together until the porosity is substantially reduced. Preferably the sintering takes place in all atmosphere of air with a partial pressure of oxygen or in oxygen e.g. not in a reducing atmosphere.
Preferably the alumina powder has a particle size of less than 3 microns and more preferably of 0.01 to 2 microns.
The dielectric loss of poly-crystalline sintered alumina has been measured by several workers and the results vary widely. For example, Ceramic Source, vol. 6 1990, American Ceramic Society (publ) reports the loss of alumina as Q=1000 at 10 GHz and 25xc2x0 C.
The highest Q previously measured in alumina at room temperature (i.e. approximately 25xc2x0 C.) is by Woode et al (R A Woode, E N Ivanov, M E Tobar and D G Blair xe2x80x98Measurement of dielectric loss tangent of alumina at microwave frequencies and room temperaturexe2x80x99 Electronics Letters, vol, 30 no. 25, Dec. 8, 1994) who measured a Q of 23,256. This Article noted that purity alone was a poor indicator of the dielectric loss tangent. We have found that although an impure alumina will give a poor Q, a very pure alumina is not a guarantee of a high Q. Pure alumina can have compounds added to it in order to assist the sintering process. These additions should not adversely influence the Q. So, for example, magnesia may be added to a very pure alumina and this will assist the sintering but will not adversely affect the Q if added in small quantities. However, adverse effects are observed with impurities such as alkali salts (sodium and potassium) and metallic elemental impurities such as iron.
It is surprising that the present invention can produce sintered alumina with a Q value greater than 30,000.